(a) Technical Field of the Invention
This invention relates to multifrequency antennas. More particularly, it relates to reconfigurable millimeterwave dipole antennas that can transmit and receive signals in multiple frequency bands.
(b) Description of Related Art
In communications applications, it is often desirable to transmit or receive signals in more than one frequency. Common dipole antennas are designed to transmit and receive in only one frequency. These antennas are typically constructed so that the length of the antenna is equal to one-half of the wavelength of the operational frequency of the antenna. At this frequency, the signal will resonate in the antenna and maximum power transfer will occur. The half-wavelength frequency also allows the antenna to transmit and receive with a broad frequency band.
Prior designs have attempted to modify the above-described dipole antenna structures to allow them to transmit and receive at more than one frequency. These designs have relied primarily on providing its frequencies at higher multiples of the half-wavelength frequency. For example, a particular design may allow the same antenna to resonant at a signal frequency equal to one full wavelength of the signal, as well as a frequency that is equal to one-half of the wavelength of the signal. However, this type of passive multi-frequency antenna has several short comings. First, the higher-multiple wavelengths do not have as large a bandwidth as the half-wavelength frequency. Also, there is no isolation between the two frequencies. If a passive dual-frequency antenna is transmitting at one frequency, it may receive interference from signals that are being received at the other frequency. In addition, these passive antennas operate best when the two frequencies are each multiples of a common half-wavelength, thus limiting the possible frequencies that a single antenna could effectively achieve.
A known approach to avoiding some of the problems of a passive multi-frequency antenna is to use two separate antennas having a switching mechanism that is used to select between the antennas. In this design, the antennas are typically positioned alongside one another. The antennas are of different lengths, and each antenna is used to transmit or receive at a separate frequency. The switching mechanism selects between the two antennas based upon which frequency is desired. This design allows any two frequencies to be used, but suffers from other drawbacks. The physical proximity of the antennas may cause them to become electromagnetically coupled to one another. This coupling could distort the effective operating frequencies of the antennas and make designing the antenna more difficult. A further problem with the switched antenna approach is the fact that two antennas take up twice as much space as a single antenna. This is a significant problem when designing dense circuits to fit in a constrained area, such as millimeterwave antennas on printed circuit boards or on integrated circuits. As more frequencies are added, thereby requiring additional antennas, this problem is compounded.
A known solution is to use a switching mechanism that changes the effective length of the antenna rather than switch in an entirely new antenna. The physical layout area required for this device would be much less than for a multiple-antenna system. U.S. Pat. No. 5,541,614, issued to Lam et. al., discloses a switched-dipole antenna for millimeterwave applications. The Lam patent discloses the use of micro-electro-mechanical (MEM) switches to add additional segments to a dipole antenna to increase the length of the antenna and allow the system to operate at multiple frequencies.